Resolve "Allow to provide strain time series in stochastic sources"

Closes #40 (closed)

• Allow to pass strain time series to StochasticPointSource instead of a generator object
• The generator argument should now be a generator object (callable), and cannot be a scalar, on StochasticPointSource and StochasticBackground
• New class methods t0_with_margin() and size_with_margin() on StochasticPointSource